1. Field of the Invention
The present invention relates to a quartz crystal vibrating piece formed from a quartz crystal wafer by etching, a quartz crystal vibrator using this quartz crystal vibrating piece, and an oscillator, an electronic device, and an electric wave clock having the quartz crystal vibrating piece wave clock.
2. Description of the Related Art
As an indispensable electronic element for the manufacture of industrial goods, there is a quartz crystal vibrator in which a quartz crystal vibrating piece is enclosed in a closed container.
A quartz crystal has a characteristic that it is expanded and contracted due to a piezoelectric effect if a voltage is applied to it and when this is incorporated in a feedback circuit of a resonance circuit, the quartz crystal carries out very accurate and stable oscillation.
By using this characteristic, the quartz crystal vibrator is used for all electronic devices including an oscillator, an electronic device such as an information communication device and a mobile device, and home appliances or the like having a clock function and a frequency control mechanism.
The quartz crystal vibrator is of a lead type in which a lead wire is arranged and of a surface mounting type in which a package is directly mounted on a printed board or the like.
In recent years, these electronic goods have been downsized and are densely packed. The quartz crystal vibrator used in such electronic goods is required to be downsized and as a result, the quartz crystal vibrating piece to be enclosed is also downsized.
As a manufacturing method of the quartz crystal vibrating piece that has been downsized for manufacturing the downsized quartz crystal vibrator, a method to form it from the quartz crystal wafer by etching has been frequently used. According to this method, a mask is formed on the opposite faces of the quartz crystal wafer by a protection film and the quartz crystal is etched by an etching solution such as a fluorinated acid and an etching gas such as CF4 so as to form a quartz crystal vibrating piece.
Here, a method of forming a quartz crystal vibrating piece will be described with reference to the drawings. FIG. 7 is a view showing an outline of a quartz crystal vibrating piece. FIG. 8 shows steps for forming a conventional tuning-fork type quartz crystal vibrating piece. A shape of a quartz crystal vibrating piece 10 is formed into the shape of a mask 60. The mask 60 is patterned by photolithography or the like. In this method, the sizes of the masks 60 to be formed and arranged on the opposite surfaces of a quartz crystal wafer 20 are the same and the positions where they are arranged are also the same.
A step 1 is a step for cleaning and baking the quartz crystal wafer 20 to be a base material of the quartz crystal vibrating piece 10.
A step 2 is a step to form an anticorrosion thin film 30 that is made of a resistance against etching of quartz crystal and is to be a base material of the mask 60.
A step 3 is a step to form a photo resist film 40.
A step 4 is a step to shield an ultraviolet ray 55 by a photo mask 50 and expose the photo resist film 40. In this case, the sizes of the photo masks 50 formed and arranged on the photo resist film 40 are the same and the positions where they are arranged are also the same.
A step 5 is a step to develop the photo resist film 40 that is exposed by the ultraviolet ray 55.
A step 6 is a step to mold the thin film 30 into the mask 60 by etchant of the anticorrosion thin film in the shape of the developed photo resist film 40. In this time, the sizes of the masks 60 formed and arranged on the opposite surfaces of the quartz crystal wafer 20 are the same and the positions where they are arranged are also the same.
A step 7 is a step to separate the photo resist film 40.
A step 8 is a step to perform etching of the quartz crystal wafer 20 by etching of the quartz crystal using the mask 60 as a mask material. A reference numeral 8a is a view showing the state in the middle of etching, and a reference numeral 8b is a view showing the state that etching is finishes to the last shape.
A step 9 is a step to stripping the mask 60.
Hereinafter, the quartz crystal vibrating piece is completed through a formation step of an electrode film or the like.
The method of forming the quartz crystal vibrating piece is a method to expose the masks used on the front and rear surfaces from the front surface side and the rear surface side and etch them. However, there is also a method to deposit the anticorrosion thin film by a transparent anticorrosion film to have a mask only on one surface, expose only from the mask surface side, and etch it.
A magnitude of frequency change per unit size is increased by downsizing of the quartz crystal vibrating piece, and at a conventional processing accuracy, the frequencies at individual vibrating pieces are largely varied. FIG. 9 is a view explaining a magnitude of frequency change per unit size of a tuning-fork type quartz crystal vibrating piece. A horizontal axis represents a size of an arm width (unit: μm) and a vertical axis represents a magnitude of frequency change per size of an arm width 1 μm (unit: Hz/μm). Here, the explanation is given taking the tuning-fork type quartz crystal vibrating piece as an example. Particularly, in a range where the size of the arm width is below 100 μm it is known that the magnitude of frequency change is rapidly increased. It is difficult to bring the matching accuracy of the front and rear surfaces to perform patterning when quartz crystal is formed by etching within 1 micrometer including repeatability in an exposure system using a mask aligner.
FIG. 10 is a view showing an example that a deviation of a mask is generated in a conventional tuning-folk type quartz crystal vibrating piece shape forming step. In the opposite surfaces-simultaneous contacting exposure in accordance with a mechanical deviation and in a projection exposure into a large area in accordance with an aberration of a lens, as shown in a step 4 of FIG. 10, a deviation is generated on the masks 60 on the front and rear surfaces and the shape of the quartz crystal vibrating piece 10 is formed as shown from the steps 8 to 9, so that variations are caused in the size due to the amount of deviation.
In addition, according to the above-described method for masking only one surface and exposing only the mask surface side, the positional accuracy of the mask is secured, however, in the exposure to the rear surface side, the size is changed due to diffraction and interference.
Further, in the pattern of the lower mask at the rear surface side, the mask is deviated in the case that an etching direction is not a direction perpendicular to the quartz crystal wafer because the pattern of the upper mask at the front surface side is transcribed in a direction perpendicular to the quartz crystal wafer to be formed.